The invention relates to electronically controlled multipoint injection devices for an internal combustion engine and more specifically an injection module belonging to such a device and comprising a rail, designed to be connected to a supply pump, and several injectors connected to the rail, each being provided with electrical control means for opening and closing it.
The injectors are manufactured to an indicated or set characteristic, which, desirably, is linear. The bold line of FIG. 1 shows an indicated characteristic which can be regarded as being typical. The quantity injected, at a constant pressure differential between the supply and the combustion chamber is essentially a linear function of the opening time. In practice, manufacturing tolerances cause variations, particularly if the injection duration is short. The curve denoted by the fine line in FIG. 1 illustrates an example of a real characteristic. In order to be acceptable, the injectors must have a characteristic whereby the variance relative to the reference characteristic does not exceed a given percentage, for example +/-5%. In order to comply with this condition, each injector manufactured is put through bench tests, during which an attempt is made by trial and error to prestress the closing spring, so that the injector is set as closely as possible to the reference characteristic. These are lengthy operations and in addition allow only limited variances to be compensated since they involve acting on the characteristic in a global manner.
U.S. Pat. No. 5,575,264 describes a first attempt to overcome the problem associated with dispersions of the characteristics. For that, an EEPROM from which the technical data of the injector can be read is attached to each injector.
An object of the present invention is to provide an injection module that will allow relatively large variations in the injector characteristics to be tolerated.
To this end, the invention proposes in particular an injection module comprising means for storing a calibration function of each of the injectors and for supplying said function in a format that can be used by a computing unit controlling the electrically controlled opening duration of the injectors.
The function to be stored is automatically determined by plotting the injected quantities for a given number of given opening durations distributed over the dynamic operating range. This function can then be stored in the form of a polynomial of a sufficient order or in map form.
Currently, fuel is injected into a chamber at a differential pressure which varies as a function of the operating parameters of the engine. This situation may be taken into account by using a dual-input map model (duration of injection and differential pressure) or by using a polynomial with two variables. In view of the fact that calibration can be performed automatically, the only manual operations being fitting and removal of the injector, it is possible to accept a very large number of measurement points with durations of injection which may vary within a very broad range, for example from 0.15 ms up to 10 ms. Since the main variable is the duration of injection, it will be sufficient to carry out tests for 2 or 3 different pressure differentials on an automatic test bench.
The stored model must accompany the injector and be taken into account when controlled by the computing unit. In one advantageous embodiment, the model is stored in a read-only memory, for example, within a complete module comprising the injection rail and the injectors, which are permanently fixed to it. The module can then be provided with a connector providing a link to the computing unit and the power amplifier which is controlled by this computing unit and opens the injectors. The connector is provided with a contact or contacts, allowing representative models of the correction relative to the indicated characteristic to be copied with a view to loading the model into the computing unit. The latter can then be programmed to take account of the corrections on each indicated duration of actuation, so as to take account of the actual characteristic of each injector.
Generally speaking, the module will also have a pressure sensor supplying an electric output signal representative of the injection pressure prevailing in the rail. The computing unit is connected to this sensor as well as to a sensor giving the pressure at the inlet of the combustion chambers, representative of the pressure prevailing in these chambers during injection. The computing unit can then take account not only of the model representative of the real characteristic as a function of the indicated duration of injection but also the corrections to be applied as a function of the differential injection pressure. The pressure sensors will generally be piezo-resistive sensors, which have the requisite robustness for a long service life under the operating conditions of an internal combustion engine. The features outlined above as well as various others will become clearer from the description of a specific embodiment of the invention given below by way of example but which is not restrictive in any respect. The description is given with reference to the appended drawings.